1. Field of the Invention
The present invention relates generally to processes for treating spent potliner material from aluminum reduction cells in a manner in which hazardous wastes are converted and recycled to useful, non-hazardous substances. More specifically, the present invention relates to a process of recovering, from spent aluminum potliner material, aluminum fluoride, reusable salts such as sodium sulfate and refractory material such as calcium feldspar which can be used to make brick products, for example. Further, large amounts of energy can be recovered from the carbon, e.g., 8000 to 9000 BTU/lb of carbon.
2. Description of the Prior Art
The Hall-Heroult process for the production of metallic aluminum dates from the 19.sup.th Century. Many refinements to the process have been made, but the basic Soderberg or pre-bake configurations using Hall-Heroult cells remain the most common processes for aluminum production throughout the world. In these processes, the bottom and internal walls of a cathode of an aluminum pot are formed with a liner of carbon blocks joined by conductive carbonaceous binder and wrapped with refractory firebricks and insulating bricks, the resulting combination being referred to as "potliner". The insulating bricks and firebricks are composed of material such as silica and alumina (aluminum oxide).
During the production of aluminum, the aluminum reduction pot is filled with a bath of alumina and molten salts. Over the three to seven year life span of an aluminum reduction pot bath, salts migrate into the potliner, thereby resulting in the deterioration and eventual failure of the utility of the aluminum cell as a cathode. During its life span, a cathodic potliner may absorb its own weight in bath salt materials. The failed potliner material is referred to as spent potliner or SPL.
When an aluminum reduction cell is taken out of service, the SPL is cooled and fractured to facilitate subsequent handling and disposal. The fractured SPL is a nonhomogenous material which contains carbon, silica and/or alumina from the insulating brick and firebricks, aluminum, significant quantities of sodium salts, aluminum salts and oxides, fluoride salts and traces of cyanides. On the average, a large aluminum smelter with a production capacity of 175,000 tons of aluminum per year will produce about 6,000-12,000 tons of SPL per year. The quantity of SPL generated annually in the United States alone has in recent years exceeded approximately 230,000 tons per year.
Because of its cyanide content, its high concentration of leachable fluoride compounds, and the high volumes of SPL produced, SPL presents a significant environmental hazard and a major burden for aluminum producers, who remain ultimately liable for the proper disposal of SPL. The SPL has long been listed as a hazardous waste by the U.S. federal and state environmental authorities. Current regulations require that SPL ultimately be treated to explicitly remove the toxic cyanide, high concentration of leachable fluoride compounds, and other characteristics which cause it to be listed as hazardous before it can be placed in a landfill disposal site.
Many different approaches have been tried over the years to convert SPL to non-hazardous materials. One major technique includes combustion or incineration of the SPL as exemplified in U.S. Pat. Nos. 4,735,784; 4,927,459; 5,024,822; 5,164,174; 5,222,448 and 5,286,274. Unfortunately, most of these processes result in an end product consisting of a glassy slag material which still contains some hazardous, allegedly nonleachable, materials.
Another process includes chemical treatment to convert SPL to non-hazardous materials. In these types of processes, as exemplified by U.S. Pat. No. 4,113,831, the initial SPL constituents are replaced with compounds which are less toxic, but which compounds are still above the hazardous listing levels established by various environmental authorities. Moreover, these residues generally have a final volume which is comparable to the volume of the input.
Another major technique of converting SPL to non-hazardous materials includes pyrohydrolysis of the SPL material. This process generally includes pyrolysis of the material in conjunction with the introduction of water to create an off-gas containing the fluoride materials as illustrated in U.S. Pat. No. 4,113,832. Such pyrohydrolysis techniques may also be used in conjunction with fluidized bed reactors as disclosed in U.S. Pat. Nos. 4,158,701 and 4,160,808. These processes also still tend to produce large volumes of waste material which must be stored in landfills and which may contain allegedly non-leachable hazardous waste. Thus, there is still a need for a process to chemically treat SPL material from aluminum reduction cells, wherein the end products of such a treatment process are all usable either within the process itself or with other commercial processes as well as secondary end products which are non-toxic to the environment and which do not include large volumes of material for the landfill or for storage.